Otitis media is the most common significant bacterial infection of childhood affecting from 10-30 percent of infants and children under 5 years of age yearly. This leads to a cumulative incidence of otitis media of 80-90 percent by 5 to 6 1/2 years of age. This investigation will determine the humoral immune response in acute and convalescent serums and middle ear exudates of patient with otitis media proven by culture to be due to Diplococcus pneumoniae or Hemophilus influenzae. Antibodies will be qualified by radioimmunoassay and indirect hemagglutination. The immunoglobulin class of the antibodies will be determined with the use of fluorescent-tagged antibody to human immunoglobulins. Antibody responses will be correlated with serial cultures of the middle ear and the long-term clinical course of patients in an attempt to characterize the natural immune response to otitis media and to relate this response to protection of the patient from subsequent bouts and otitis media due to the same or other serotypes of these two bacteria. Since highly purified capsular polysaccharides from the pneumococcus and H. influenzae type b are presently under study as vaccines, characterization of the natural immune response to localized infection by these bacteria will serve as an important basis for comparison of the response to vaccination.